The present invention relates to miniature electric fuses, useful in many electrical apparatus such as televisions, computers and the like. The invention particularly relates to the connection of the lead wires to the body of the fuse and more particularly is directed to an end capless fuse.
Known miniature fuses typically comprise a gas pressure resistant, hollow, open ended, insulating sleeve, which might be of glass, ceramic, or other electrically non-conductive material. A fusible element, adapted to melt on electrical overload, extends through the sleeve and is electrically connected to end caps that are affixed at the ends of the sleeve or to leads ending at the end caps. The electric leads to the fuse are either attached to the end caps and through those end caps to the fusible element in the sleeve, or the leads extend through the end caps to the fusible element.
Examples of a miniature fuse with end caps are seen in U.S. Pat. Nos. 4,746,784 and 4,540,969. An example of a lead wire which is connected to a fuse is found in U.S. Pat. No. 2,916,587. An unusual end cap arrangement is shown in U.S. Pat. No. 4,532,489.
When the electric circuit in which the fuse is connected suffers an electric overload, the fuse wire melts and perhaps vaporizes, solder inside the fuse sleeve may do the same, solder flux may vaporize and the gas or air in the fuse becomes heated. The gas pressure inside the fuse sleeve increases and rapidly. The temperature, particularly of a short circuit, may be high enough to soften the solder, weakening its hold on the leads. The gas pressure may apply enough pressure to the end caps and/or to the leads at the end caps to blow them out of the sleeve.
Often the assembled miniature fuses are encapsulated in a covering material. This material will block or at least inhibit the blowing out of the leads and end caps. Typically, it also serves as electric insulation of the covered elements. For example, in one technique known for years, miniature fuses are placed in a mold, and molding material is transfer molded around them. Transfer molding materials, which typically are known molding epoxies, are formulated so that they will not adhere to the metal end caps or to the leads or even to the nonconductive sleeve around the fuse wire. Such molding material therefore will not prevent a lead to a fuse and sometimes even the end cap on the fuse sleeve from blowing out of the covering material where elevated pressure developing in the sleeve, unless the fuse is so designed to permit the molding material to flow around an angle or a projection or into an undercut or notch. It is known that typically non-adherent molding materials, which are molded around a lead, will not hold the lead against moving through the molding material unless there is a knurl, notch, upset, enlargement, crimp, or a like misshapen section, formed in the lead, so that the molding material can flow into or around that section of the lead and lock onto the lead when the molding material cures.
Miniature fuses are intended to be extremely simple in structure and inexpensive to manufacture. One way to accomplish these objectives is to make the miniature fuse end capless. In such a fuse, the lead is soldered directly to the ceramic or glass sleeve of the fuse. An arrangement of a lead attached directly to a fuse wire is shown in Japanese Patent Publication Nos. 53 35144 and 52 22750. For enabling attachment of the leads, the ceramic sleeve may be metallized, which substantially is plating, at both of its ends, to enable the leads to be soldered to the metallizations and also to enable the ends of the fuse wire to be soldered to the metallizations. Then the assembled soldered fuse is placed in a mold and molding material is molded around both the leads and the sleeve. If each lead is simply a straight wire or is a stiff pin extending from the end of a lead and to the fuse, the lead is held in place at the sleeve only by the strength of the solder joint between the lead and the metallized ceramic. Temperature, or outward pushing forces caused by overload or short circuit conditions, or physical pulling on leads during installation, can weaken or melt the solder joints so that the leads can be easily pulled out or can be blown out of the fuse assembly. Some effective means is needed to provide a capless fuse wherein the lead is securely held to the fuse sleeve rather than to an end cap over the sleeve.